It is known in the art, according to which a member or a device for decreasing pressure variation (hereinafter also referred to as a noise decreasing member) is provided on a product surface in order to decrease aerodynamic sound (also referred to as noise) generated in a blower device for blowing out conditioned air or cold air, for example, as disclosed in the following patent publications:
(1) Japanese Patent Publication No. H02-253000;
(2) Japanese Patent Publication No. H05-026762;
(3) Japanese Patent Publication No. H07-225048;
(4) Japanese Patent Publication No. 2003-161295; and
(5) Japanese Patent Publication No. 2006-159924.
For example, a structure for providing a plumate or wire-rod-shaped projections or furry and fibrous members is described as the noise decreasing member, in the above patent publications.
In a case that one of the noise decreasing members disclosed in the above patent publications is applied to an air duct of an air conditioning apparatus for a vehicle, wherein the air duct has a bended portion, it is not possible to achieve a sufficient noise decreasing effect. It is difficult to use such noise decreasing member in view of an industrial application, because it may cause a decrease of performance and/or an increase of manufacturing cost.
When the technology disclosed in the above patent publications (1) or (3) is applied to the air conditioning apparatus for the vehicle, it may be necessary to provide a lot of projections in a plume structure on an inner surface so as to break a vortex flow into multiple smaller portions, or it may be necessary to provide small fibrous members in a furry condition (that is, a kind of hair-implant process) so as to softly receive the vortex flow by cushioning action.
A part (a member) of the air conditioning apparatus for the vehicle, which forms a wall of an A/C casing, is generally made of resin by a molding process. Therefore, it may become necessary to add the above kind of the hair-implant process in the resin-molding process. It may cause an increase of the manufacturing cost.
In addition, when the fibrous members may come off due to a secular change, not only the noise reducing effect may be decreased but also the air duct of the air conditioning apparatus may be blocked by such fibrous members or the fibrous members may be blasted off from a duct opening into a passenger compartment to thereby provide an uncomfortable feeling to a vehicle passenger.
In addition, in a case that the technology of the above patent publication (2) is applied to the air conditioning apparatus for the vehicle, it is necessary that a boundary-layer flow partly transits to a turbulent boundary layer in an area adjacent to air blow-out ports and air duct openings. Therefore, it may be necessary to provide a facilitating member on a surface which is in contact with the air flow, so that the boundary-layer flow partly transits to the turbulent boundary layer.
However, since the air conditioning apparatus for the vehicle is composed of multiple complicated air passages, which have a lot of bended portions in the inside thereof, the noises are also generated at such portions other than the air blow-out ports and the air duct openings. Therefore, even when the noise generated in the area adjacent to the air blow-out ports and/or the air duct openings can be decreased, the noise as a whole can not be still sufficiently decreased. Since cold air and hot air is mixed with each other in order to adjust temperature of the air in the air conditioning apparatus for the vehicle, air currents in the inside of the air conditioning apparatus are largely disturbed and those air currents are in a condition of the turbulent flow. If the above facilitating member was provided, the turbulence of the air currents would be further increased and the noise would be increased on the contrary.
In addition, in the case that the noise decreasing members of the above patent publications (1) to (3) were provided in various portions of the air passage for the purpose of decreasing the noise, the noise decreasing members would become resistance for the air flow to thereby cause another problem that a flow rate may be decreased in accordance with an increase of the area, in which the noise decreasing members are provided.
It might be possible to increase rotational speed of a blower unit and to supply the air at a higher pressure so as to compensate the decrease of the flow rate. However, the noise will be correspondingly increased. Namely, the aerodynamic sound for the unit flow rate may be increased.
In addition, in the case that the technology of the above patent publication (4) was applied to the air conditioning apparatus for the vehicle, it would be possible to decrease the noise, which will be generated at portions, such as forward portions of ribs, inside wall surfaces of the bended portions or the like, at which the air currents are separated. However, on the other hand, it would not be possible to decrease the noise which will be generated when the air currents separated at the bended portions or the forward ends of the ribs collide against wall surfaces around them, or the noise which will be generated when the air currents collide against an outer wall surface of the bended portion. Therefore, it is not possible to decrease the noise as a whole. The technology of the patent publication (5) is applied to a reduced portion of the air passage. Therefore, it is not possible to decrease the noise, which will be generated at other portions than the above reduced portion. In other words, even according to the technology of the patent publication (5), it is not possible to decrease the noise as a whole, either.